1. Field of the Invention
The present invention relates to a method of manufacturing a high strength valve spring for a vehicle engine and a high strength valve spring manufactured using the same. More particularly, the present invention relates to a high strength valve spring for a vehicle engine and a method of manufacturing the same, which can prevent damage and deterioration of the spring, particularly wherein the spring is formed using a high strength wire rod.
2. Description of the Prior Art
A valve spring for a vehicle engine interlocks with a device that connects a valve with a cam shaft, such as a cam shaft and a swing arm in an engine, a tappet, and the like, and adjusts opening and closing of an intake valve and an exhaust valve.
A process of manufacturing a valve spring for an engine in the related art, as illustrated in FIG. 1, includes a forming process and a cutting process for forming a wire rod into a spring shape, a residual stress removal heat treatment process for removing a residual stress occurring on inner and outer surfaces after forming, a shot peening process for applying a compression stress to the surface with fine ball particles to increase fatigue strength, and a hot setting process for applying plastic deformation in advance to increase deformation resistance.
A wire rod that has been used to manufacture a valve spring in the related art is mainly a Si—Cr steel wire rod having tensile strength of 1900 MPa. This wire rod is formed into shape form of a spring using a coiling jig as illustrated in FIG. 2. As shown, a cutting blade descends vertically to cut and separate an end portion of the formed spring.
After forming the spring, a residual stress occurs on an inner surface and an outer surface of the spring. Specifically, a compression residual stress occurs on the outer surface, and a tensile residual stress occurs on the inner surface. Since the tensile residual stress that occurs on the inner surface lowers the fatigue strength of the spring, a residual stress removal heat treatment process is performed to remove the tensile residual stress.
The residual stress removal heat treatment is maintained at 410 to 420° C. for 20 to 30 minutes.
After the residual stress removal heat treatment, a shot peening process for artificially generating a compression residual stress on the surface of the valve spring is performed to increase the fatigue stress.
The shot peening is performed with SWRH82A as a material. In particular, shot peening using a shot ball having a diameter of 0.6 mm is first carried out for 40 minutes, followed by shot peening using a shot ball having a diameter of 0.3 mm for 20 minutes.
After the shot peening, a hot setting process is performed. The hot setting process is a process that applies, in advance, plastic deformation to the spring, and thus prevent deformation during driving in an engine.
The hot setting process compresses the valve spring up to the maximum displacement for 1.0 to 1.5 seconds after maintaining the spring at 220° C. to 230° C. for 10 to 15 minutes. At this time, a plastic deformation occurs, and resistivity against deformation during the actual driving of the valve spring is increased by work-hardening.
However, in applying the conditions of the valve spring manufacturing process in the related art to a high strength wire rod, a number of problems occur.
First, if the conventional jig (hereinafter referred to as a “fixed jig”) is applied during the formation of the spring, deformation occurs due to friction, and the valve spring is broken or fine cracks occur therein during spring formation.
Also, if the conventional cutting method (hereinafter referred to as “vertical cutting”) is used, in the case of a high strength wire rod, the impact toughness is relatively low. As such, cracks occur on portions of the spring other than the cut surface due to the impact during cutting.
Further, if the conventional temperature and time conditions are applied in the residual stress removal heat treatment process, while the residual stress may be removed, the hardness and the strength are lowered due to mutual reactions of the alloying elements. As a result, these advantages of the high strength valve spring are lost.
Still further, if the conventional conditions are applied during the hot setting process, the deformation resistance effect of the process is decreased.
In order to solve the above-described problems, according to an embodiment of the present invention, damage to the spring is prevented by improving the forming jig during the forming and the cutting method. Further, the deterioration in hardness is prevented and the residual stress is removed by applying optimum temperature and time conditions during the residual stress removal heat treatment process. Also, by applying optimum hot setting temperature and time conditions, the deformation resistance is increased.